A functionally gradient material (FGM) is an anisotropic composite material and can be a metal-ceramic material. A gradient in composition and/or microstructure is deliberately introduced into the material which results in a gradient in composite properties. Depending upon the application of the FGM, the gradient can be created in a continuous or stepwise fashion. FGMs find use in aerospace applications, in microelectronics, and in medical applications.
FGMs can be prepared by chemical or physical vapor deposition (CVD/PVD), conventional powder metallurgy processing, plasma spraying or Self Propagating High Temperature Synthesis (SHS). However, these methods are not ideal. For example, conventional powder metallurgy processing involves consolidation processes that typically require binders, such as organic binders, which must be removed during some point of the process, usually by heating of the green composite before sintering. Thermolysis of the binders generates volatile byproducts, removal of which can be costly and must be accomplished in a carefully controlled manner if cracking, deformation or bloating of the part is to be avoided. Also, shrinkage of individual layers can occur, especially when top and bottom layers have large differences in density.
FGMs can also be prepared by the gelcasting method. For example, U.S. Pat. No. 6,375,877 to Lauf et al. discloses suspending two different phases into a mold, thereby creating a compositional gradation within the object, and the slurries are then gelled. The individual slurries are selected such that they have comparable sinterability so that uniform shrinkage occurs during sintering. Thus, the method is not widely applicable. In another method, disclosed in U.S. Pat. No. 6,776,860 to Arai et al., two ceramic composites are bonded together by dispersing a bonding ceramic between them and then sintering to the desired porosity.
Thus, there exists a need for methods for the production of functionally gradient material which avoids the use of those types of binders, which, when removed by thermolysis, generate volatile byproducts whose removal is relatively costly and often results in degradation of the properties of the material being formed. This method should also provide a means for controlling shrinkage of functionally gradient material during processing.
Recently, Cold Spray Process (CSP) (U.S. Pat. No. 5,302,414 to Alkhimov et al. (1994), McCune et al. “An Exploration of the Cold Gas-Dynamic Spray Method for Several Materials Systems,” J. Thermal Spray Science and Technology, C. C. Berndt and S. Sampath, Ed., ASM International, 1995, pp. 1-5, and Dykhuizen et al. “Gas Dynamic Principles of Cold Spray”, J. Thermal Spray Tech., 7:205 (1998)) has been used for the deposition of coatings and for the production of FGMs. CSP uses a supersonic gas jet (velocity of 5000 km/h) to accelerate solid fine powders (micron size) of various materials above a critical velocity at which particles impact, deform plastically and bond to the substrate to form the coating. CSP does not use plasma, combustion processes or any other thermal source, therefore, the spray environment is very clean and the material is not exposed to high temperatures. The present invention discloses the use of CSP for forming the shaped bodies and the use of Field Assisted Sintering Technique (FAST) for sintering the body.